This invention relates to a semiconductor device such as, for example, a Zener diode.
In a Zener diode which is utilized as a voltage regulator device, the breakdown voltage of a PN-junction is exploited as a constant voltage. The breakdown voltage of the PN-junction varies depending upon an impurity density deviation, crystal defects, etc. Accordingly, when the junction area of the PN-junction is large, the breakdown voltages of individual minute junction parts within the junction area are different due to the presence of impurity density deviations or crystal defects in the respective minute junction parts, and the Zener diode exhibits a soft breakdown characteristic.
On the other hand, the Zener (tunneling) phenomenon predominates in a Zener diode whose breakdown voltage is below 5 V, whereas the avalanche breakdown phenomenon predominates in a Zener diode whose breakdown voltage is above 5 V. Whichever phenomenon may be predominant, the Zener diode exhibits the soft breakdown characteristic when it utilizes a large PN-junction area. Simultaneously, noise is generated by the minute breakdowns at the minute junction points, so that the noise characteristic becomes inferior.
Thus, in order to obtain a Zener diode having a hard breakdown characteristic and a low noise characteristic, it is desirable to make the junction area of the PN-junction small.
Further, when the current density of D.C. current which flows through the PN-junction upon the breakdown of the PN-junction becomes excessive, the junction temperature of the PN-junction rises and the thermal carrier creation of electrons and holes acts as positive feedback, so that the current density increases more. On account of the excessive increase of the current density of D.C. current, current crowding occurs on one place of the PN-junction to give rise to the thermal breakdown or secondary breakdown of the PN-junction, and the current crowding place of the PN-junction is permanently destroyed.
The inventors' study has revealed that the permanent destruction of the PN-junction due to the thermal breakdown or the secondary breakdown occurs even when a non-D.C. current at an excess current density flows through the PN-junction for a comparatively short time. The PN-junction of the Zener diode accordingly undergoes the permanent destruction due to a non-D.C. voltage such as surge voltage of an impulse nature from the human body of a circuit assembling worker, etc. or an A.C. voltage leaking from a soldering iron such as is applied when the Zener diode is packaged on a circuit board, or when an abnormal surge voltage is applied to a finished circuit after the Zener diode has been packaged on the circuit board.
According to the inventors' research, it has been experimentally revealed that the critical current density at which the permanent destruction of the PN-junction of the Zener diode occurs varies depending upon the device structure and dimensions of the Zener diode, the heat radiating structure of a case or envelope, and the pulse width of an impulsive surge voltage or the frequency of a leakage A.C. voltage and that the critical current density has a value of approximately 16.times.10.sup.6 (mA/cm.sup.2) when the voltage applied to the Zener diode is an A.C. voltage. According to the inventors' study, it has been revealed that the point of the destruction attributed to the application of such abnormal voltage of an impulse nature or an A.C. voltage nature lies outside the characteristic curve of the maximum allowable power dissipation P.sub.dmax during the D.C. operation of the Zener diode.
To the end of intensifying the characteristic of endurance against the permanent destruction of the PN-junction ascribable to the application of the abnormal voltage of an impulse nature or an A.C. voltage nature, it has been considered to lower the current density by enlarging the junction area of the PN-junction.
However, with the large junction area of the PN-junction, the Zener diode exhibits the soft breakdown characteristic and the inferior noise characteristic even though the endurance characteristic against the permanent destruction is enhanced.